The following summary is set forth, by way of example, in the context of a “soft-X-ray” (SXR) microlithography system as an exemplary X-ray optical system.
As active circuit elements in microelectronic devices have become progressively smaller, the diffraction-limitations of current optical microlithography systems have become increasingly burdensome. Consequently, a large effort is in progress to develop a practical “next-generation” lithography (NGL) technology. An especially promising NGL approach is performed using an X-ray-wavelength beam as the lithographic energy beam. In this regard, SXR (EUV) microlithography systems are under active development, which utilize a wavelength (11-14 nm) that is approximately {fraction (1/10 )}the shortest wavelength used in conventional optical microlithography. EUV lithography (EUVL) is capable of transferring fine patterns having elements as small as 70 to 30 nm. In any event, since a much shorter wavelength is used for the lithographic energy beam, substantially improved resolution of pattern elements is possible.
For X-ray wavelengths including EUV wavelengths, the refractive index is very close to unity for all known materials that have been considered for use in making refractive lenses for such wavelengths. Also, the reflectivity of X-radiation from most known materials is very low. Thus, it currently is impossible to construct X-ray optical systems employing conventional refractive and reflective optical elements. However, certain types of mirrors have been developed that exhibit rather high reflectivity to incident X-radiation. These mirrors, called “multilayer-coated” mirrors, have a surficial multilayer coating that reflects incident X-radiation based on constructive optical interference. Specifically, multiple layers of two respective materials (exhibiting different refractive indices and being high-Z and low-Z materials, respectively) are laminated to the surface of a mirror substrate in an alternating manner. The total number of each type of layer normally is several tens to several hundreds of layers. The multilayer coating presents weakly reflective interfaces between each pair of layers, and the respective thicknesses of the layers are configured to provide constructive interference of the reflected radiation from the various interfaces to yield a high net reflectivity of incident radiation. Hence, the multilayer coating is termed an interference coating. X-ray optical systems, notably projection-optical systems, as used in EUV microlithography systems typically comprise multiple multilayer-coated mirrors.
These reflective-optical systems are also termed diffraction-limited optical systems. Unless each multilayer-coated mirror of the system produces a sufficiently small wavefront error of reflected light, obtaining a desired optical performance consistent with target design rules is impossible. A standard parameter used for denoting the permissible wavefront error is the wavefront-error root-mean-square (RMS) value, which normally is one-fourteenth of the wavelength of the lithographic energy beam. A condition of λ/14 corresponds to a Strehl ratio (the ratio of the peak field amplitude in the focus of an optical element to the diffraction-limited amplitude) of 80% or greater. Projection-optical systems used in actual EUV microlithography systems typically are manufactured to have an even lower wavefront error than λ/14.
The typical wavelength of the energy beam used in EUVL is approximately 13 nm. At such a wavelength the wavefront error (WFE) RMS is 1 nm or less. If “m” denotes the number of mirrors used in the optical system, then the permissible form error (FE) for each mirror of the system is expressed by:FE=WFE/2/(m)1/2·(RMS) 
The reason that WFE is divided by two is that both the incident light and the reflected light are affected by form error. Consequently, the permissible form error (FE) for each mirror in a diffraction-limited optical system is obtained from the wavelength λ and m as follows:FE=λ/28/(m)1/2·(RMS) 
For example, with λ=13 nm and m=6 mirrors, the permissible form error for a single mirror is 0.19 nm (RMS). This value is extremely small. It is extremely difficult to manufacture a multilayer-coated mirror (especially an aspherical mirror) having this precision. It also is extremely difficult to avoid mirror-surface deformations that arise from mounting the mirrors in an optical column. Furthermore, no matter how high the precision with which a mirror is fabricated, the profile of the reflective surface of the mirror is never ideal; it always has some residual form error. Mounting the mirrors to, for example, a rigid metal frame can cause significant changes to the respective profiles of the reflective surfaces, resulting in a larger than expected form error for the overall system.
Conventionally, multilayer-coated mirrors are supported using multiple identical support devices situated at respective locations around the circumference of each mirror. The respective points of contact of a support device with the mirror normally are spaced equi-angularly around the circumference of the mirror. The most common scheme is to use three support devices that hold the mirror by local application of compressive pressure to respective points located at 120° intervals about the center of the mirror. Since the same compressive force is applied to the mirror side at each contact point by the respective support devices, local deformation (due to the limited elasticity of the mirror substrate) of the reflective surface can arise in the vicinity of one or more of the contact points, resulting in a degradation of the optical performance of the mirror. Other mirror form errors can arise during fabrication of the mirror, but fabrication-related form errors tend not to occur with regularity or predictability. Rather, fabrication-related form errors tend to differ from mirror to mirror. Whenever identical support devices are used to support each of such mirrors having different form errors, reflective-surface form errors can arise that are non-uniform around each mirror.
In addition, whereas changes in the reflective-surface curvature of mirrors and other rotationally symmetrical elements can be eliminated by performing optical-system adjustments, non-rotationally symmetrical and/or non-uniform deformations cannot be eliminated by making optical-system adjustments.
As discussed above, the profile of the reflective surface of a multilayer-coated mirror can change after the mirror is mounted in an optical column. The change can arise either from non-uniform form errors occurring on the reflective surface during mirror fabrication and/or from non-uniform deformations arising from the application of non-uniform forces in the circumferential direction to the mirror as the mirror is being supported by a rigid frame in the optical column. Whenever microlithography is performed using an optical system comprising one or more such compromised mirrors, the accuracy and precision of pattern transfer is adversely affected.